The invention relates to an apparatus for withdrawing liquid from a receptacle closed by a cover.
To ensure sterility and to reduce difficulties connected with solvent evaporation, reagents and samples for automatic analysis systems are often provided in receptacles which are closed by a resilient septum or by a plastic cover. Withdrawal of the liquid contents of the receptacle is accomplished by means of a withdrawal cannula which is connected by way of flexible piping to a pump and which pierces the cover in order to dip into the body of liquid.
To reduce the risk of contamination, analysis samples are often dispatched or delivered in receptacles of this kind, and specimens are withdrawn by means of a withdrawal apparatus with the use of an appropriate cannula.
To ensure that the resilient cover, which is often in the form of a rubber diaphragm, is punctured neatly the cannula must have a special high-precision sharpening and so is relatively expensive. Even then, it is often impossible to prevent the cannula from becoming blocked by dislodged pieces of the diaphragm. Further, the force required to puncture the diaphragm is relatively high despite the special sharpening of the cannula tip, and so the pivot arms and bearings experience elevated forces. Yet another disadvantage is that the diaphragm engages sealingly with the cannula so that when liquid is removed a negative pressure is generated in the receptacle which has disadvantageous consequences; for example, the negative pressure may impair sampling accuracy when sample quantities are small.
It is an object of the present invention to disclose an improved apparatus designed to avoid the above-mentioned disadvantages.